Why Does Don Juan Pond in Antarctica Never Freeze?

The Short Answer

Don Juan Pond in Antarctica stays liquid at temperatures as low as −50°C because it is saturated with calcium chloride brine — making it roughly twelve times saltier than the ocean and giving it the lowest freezing point of any natural body of water on Earth.

Where Is Don Juan Pond?

Don Juan Pond sits in Wright Valley, part of Antarctica’s McMurdo Dry Valleys — a stretch of terrain so cold and arid that it is often compared to the surface of Mars. The Dry Valleys receive almost no snowfall and experience relentless katabatic winds that strip away any moisture, making them one of the most inhospitable places on the planet. Despite this, a small, shallow pool of liquid water has persisted here year-round for as long as scientists have observed it.

The pond was discovered in 1961 and named after the two helicopter pilots — Donald Roe and John Hickey — who first landed beside it. It is remarkably unimpressive in appearance: roughly 100 metres across and only about 10 centimetres deep at its maximum. You could wade from one side to the other in seconds.

What Makes It So Salty?

Ordinary seawater is about 3.5% salt by weight. Don Juan Pond contains calcium chloride brine at concentrations approaching 40% by weight — a level so extreme that the chemistry of the water itself is fundamentally altered. Calcium chloride is hygroscopic, meaning it actively draws moisture from the surrounding environment. Scientists believe the pond is fed by a combination of groundwater seeping through calcium chloride-rich rock and surface moisture absorbed from the cold, dry air.

This extraordinary salt concentration is the key to the pond’s impossible behaviour. Salt disrupts the hydrogen bonding between water molecules that causes freezing. The more dissolved salt present, the lower the temperature must drop before ice can form. For Don Juan Pond, that threshold falls below −50°C — colder than a typical Antarctic winter night.

Why Do Scientists Care So Much?

The implications reach far beyond one unusual puddle in Antarctica. Mars has surface and near-subsurface temperatures that regularly sit around −50°C, and recent radar observations by the European Space Agency’s MARSIS instrument have detected signals beneath the Martian south polar ice cap that some researchers interpret as possible subsurface liquid water. If calcium chloride or perchlorate brines exist in Martian geology — and there is good evidence that perchlorates are widespread on Mars — the physics demonstrated by Don Juan Pond suggests those brines could remain liquid even now.

This matters enormously for the question of life. Liquid water is considered the single non-negotiable requirement for life as we understand it. Don Juan Pond itself is largely considered too harsh for microbial life — the water activity is too low for most organisms to function — but it proves that extreme brine chemistry can maintain liquid water in conditions that would otherwise be solid ice.

The Bigger Picture

Don Juan Pond is a reminder that Earth still holds genuinely alien environments. A body of water that defies freezing, in one of the driest deserts on the planet, at depths a child could stand in — and it is quietly pointing scientists toward one of the biggest questions in human history: are we alone in the universe? The physics of one impossible Antarctic pond suggests the answer might be more complicated, and more hopeful, than we once assumed.